1. Field of the Invention
The present invention relates primarily to a low noise amplifier/mixer circuit used in conjunction with a frequency synthesizer in the front end of a RF receiver system. More particularly, the present invention relates to low noise/mixer circuits that are coupled to a frequency synthesizer in a RF system in a manner that enables the bias current of the low noise amplifier to be reused for the bias current of the oscillators within the frequency synthesizer, thereby producing a circuit that is highly energy efficient and is readily adapted to monolithic integration.
2. Prior Art Statement
RF systems, such as those used for mobile telephones, are becoming more sophisticated every year. As the number of mobile telephones increases, the frequencies used to carry the mobile telephone signal are becoming increasingly tightly spaced. For example, the Groupe Speciale Mobile (GSM) system, which acts as a mobile telephone standard in Europe, operates with 124 channels at 200 kHz spacings. The more complex IS54 mobile telephone application, operates at 800 channels with only 30 kHz spacings. The tight channel spacings typically associated with mobile telephone systems require the transceiver within the mobile telephone to be able to transmit and receive signals on small bandwidths without creating or receiving interference outside of the target bandwidth.
Accompanying the increase in sophistication in the transmission and reception of mobile telephone signals has come a trend to make mobile telephones much smaller and lighter. One of the largest and heaviest components of a mobile telephone is the rechargeable battery pack. As telephones are made smaller and lighter, so are the battery packs. This typically means that the battery packs have less power and a shorter functional life in between recharges. Attempts have therefor been made in the prior art to produce RF front end components that have reduced power requirements. Such prior art is exemplified by U.S. Pat. No. 5,361,044 to Norimatu et al., entitled PHASE LOCKED LOOP FREQUENCY SYNTHESIZER.
One of the primary solutions used in creating mobile telephones that are small, lightweight and do not require a lot of power has been the increased use of monolithic integration within the circuitry of the telephone. Obviously by manufacturing a large number of required circuits on-chip, great savings can be produced in size, weight and power consumption. There are many technologies available for forming circuits using monolithic integration. However, digital CMOS integration processes are particularly advantageous because digital CMOS technologies offer both a mature design environment and a relatively rapid prototyping ability.
In the prior art, the use of monolithic integration in the front end of a RF system has been problematic due to sensitivity of such circuits to noise both within and outside the narrow bandwidths of the individual transmission channels. As mobile telephone systems become more sophisticated and the transmission channels more tightly spaced, noise concerns have become such a problem that it has inhibited the use of full integration. For instance, passive resonators (i.e. LC tanks) cannot be effectively manufactured using monolithic integration techniques. Consequently, integrated oscillators are manufactured without passive resonators, thereby producing a circuit that has a very large phase noise. Consequently, the use of integrated oscillators is typically limited to applications, unlike mobile telephone communications, where noise is less of a concern. A real need therefore exists in the art for a way to manufacture mobile telephone circuitry using monolithic integration in a manner that reduces noise to acceptable tolerances.
It is therefore an object of the present invention to provide an RF system front end that has reduced noise characteristics and can be manufactured using monolithic integration.
It is a further object of the present invention to provide a RF system front end having greatly reduced power requirements.
It is yet another object of the present invention to provide a low noise amplifier/mixer/frequency synthesizer circuit on the integrated level having reduced phase noise and band harmonics.